On the topic of Mab having never been able to lie before even if she'd been human because of the "the stars will fall from the...blah...blah...blah" line, it's irrelevant.
It's a matter of ambiguity versus specificity. If she had said that and added "again" at the end of the line, it would not be ambiguous. We would KNOW that she had lied before at some time. If she had included at the proper point in the statement "ever," it would not be ambiguous. We would KNOW that she had never lied before at any time. As it stands, by adding neither term nor any term synonymous with them, the entire statement is absolutely ambiguous as to whether or not she ever has lied BEFORE making that statement...most presumably during a time when she may not have actually been Fae. By her statement we can only derive certainty that she WILL NOT lie from then...we can not determine from it that she HAS NOT lied before then. Considering the Fae propensity toward wordplay and Mab's personal affinity for it, it's not at all reasonable to assume such a certainty without knowing with additional certainty that she has in fact always been Fae.
Now...as for the theory itself of Molly being Mab...I find it highly interesting, and I can't really disprove it. But, it doesn't quite feel right to me. Possible yes, but I'm gonna say my opinion is that it probably isn't the case. Sure be interesting if I'm wrong though...poor Harry.
Oh, one last thing to add to the mix...Harry did dump a pitcher of ice water over her when she tried to get in his pants the first time...
